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Intertidal seaweeds are periodically exposed to air where they experience a variety of potentially stressful environmental conditions, including nutrient limitation, high light, high and low temperature, desiccation, and osmotic stress. This paper considers the current understanding of stress tolerance in intertidal seaweeds and discusses ways in which future research could increase our understanding of the role of environmental factors in the ecology and physiology of these algae. We believe research is required in at least three areas. 1) Laboratory physiological studies have established that correlations exist between stress tolerance and the vertical distribution of species. However, little information is available on the importance of stress in determining community structure in nature. Field experiments are essential to relate the impact of single or multiple stresses on the survival, growth, and reproductive output of macroalgae. In paticular, it is necessary to clarify the role of sublethal stress in determining the outcome of competitive interactions. 2) With the exception of obvious lethal effects or damage associated with extreme environmental conditions, such as unusually hot and dry weather, it is difficult to evaluate the occurrence and severity of stress in natural populations of seaweeds. There is a need to develop molecular and biochemical markers specific for individual stresses or groups of stresses to allow the unambiguous and direct determination of stress in situ. 3) Despite the apparent importance of stress in intertidal seaweeds, we are largely ignorant of the mechanistic basis of tolerance. The application of currently available tools of molecular and cell biology to the investigation of stress‐induced transcriptional and translational changes could enormously increase our understanding of both the sites of, and pathways involved in, stress tolerance. In summary, there are numerous unanswered fundamental questions about the stress tolerance of intertidal seaweeds, providing opportunities for research ranging from field ecology to molecular biology and biochemistry.

Abstract. Sporophytes of the brown alga Laminaria saccharina (L.) Lamour grown at 15°C contained significantly more chlorophyll a (chl a) than did similar plants grown at 5°C. The increase in chl a in 15°C plants was due to increased numbers of photosystem II reaction centes, and possibly to increased photosynthetic unit size, compared with 5 °C plants. These changes were associated with increased e values (photosynthetic efficiencies) in 15 °C-grown L. saccharina relative to 5 °C-grown plants. The changes in e together with reduced respiration rates allowed 15 °C-grown L. saccharina to achieve net photosynthesis and light-saturated photosynthesis at a lower photon fluence rate (PFR) than 5 °C plants when both groups were assayed at the same temperature (15 °C). The photon fluence rates necessary to reach the compensation point and achieve light-saturated photosynthesis (I c and Ik, respectively) increased with increasing incubation temperature in L. saccharina grown at both 5 and 15 °C. However, acclimation responses to growth temperature compensated for the short-term effect of temperature on I c and I k. Consequently, plants grown at 5 and 15 °C were able to achieve similar rates of light-limited photosynthesis, and similar I c and I k values at their respective growth temperatures. These responses are undoubtedly important for perennial seaweeds such as L. saccharina, which frequently grow in light-limited habitats and experience pronounced seasonal changes in water temperature.

The effect of growth temperature on photosynthetic metabolism was studied in the kelp Laminaria saccharina (L.) Lamour. Photosynthesis was subject to phenotypic adaptation, with almost constant photosynthetic rates being achieved at growth temperatures between 0 and 20° C. This response involved: (1) an inverse relationship between growth temperature and photosynthetic capacity, (2) a reduction in the Q10 value for photosynthesis of L. saccharina grown at 0 and 5° C compared with 10, 15 and 20° C grown sporophytes, and (3) an acquired tolerance of photosynthesis to temperatures between 15–25° C (which inhibited photosynthesis in 0 and 5° C grown L. saccharina) in sporophytes grown at 10, 15 and 20° C. The physiological basis of these adaptations is discussed in terms of observed changes in activities and kinetics of the Calvin cycle enzyme ribulose‐1, 5‐bisphosphate carboxylase (oxygenase) and efficiency of light harvesting‐electron transport systems.

Mastocarpus stellatus and Chondrus crispus are morphologically similar red seaweeds that co‐occur on rocky intertidal seashores in the Northern Atlantic. Mastocarpus stellatus grows higher on the shore and is more tolerant of environmental stress, caused by factors such as freezing and desiccation, than C. crispus. Here we report a correlation between reactive oxygen metabolism and stress tolerance, which suggests that reactive oxygen metabolism may play a role in stress tolerance of intertidal red seaweeds. Mastocarpus stellatus scavenged added H2O2 slightly faster, and was more resistant to oxidative stress induced by addition of H2O2 and Rose Bengal, than C. crispus. These data were consistent with higher levels of ascorbate and β‐carotene and higher activities of catalase and glutathione reductase, in M. stellatus. Tocopherol content and activities of superoxide dismutase and ascorbate peroxidase were similar in both species. Activities of reactive oxygen scavenging enzymes generally increased with tidal height in M. stellatus; this was, however, not a consistent trend in C. crispus.